Under terms of the exchange offer, Express Scripts will offer $29.25 in cash and 0.426 shares of stock for each share of Caremark. Based on Friday's closing stock prices, the company said its bid represents a 7% premium over the current CVS proposal.

Caremark's board has voted against Express Scripts' offer.

The exchange offer will expire Feb. 13. Caremark said it was taking the action "in light of the Caremark Board's rejection of and refusal to even discuss our superior proposal."

"CVS remains committed to closing its merger with Caremark in the first quarter of 2007," a company representative said.